Television transmitting tube



Dec. 23, 1941. R. THEILE TELEVISION TRANSMITTING TUBE Filed Jan. 9, 1959 2 Sheets-Sheet 1 TO AMPLIFIER TO MMPL/F/ER lNVENTOfi RICHARD ,7HEILE A'T'ToRNEY Dec. 23, 1 941. R. 'THEILE 2,266,920

TELEVISION TRANSMITTING TUBE Filed Jan. 9, 1959 2 Sheets-Sheet 2 SCANNING TIME-t TIME ELEMENT CURVE 0F AVERAGE VALUES W C- SMALL c- LARGE 0- MEDIUM TIME-t INVENTOR RICHARD T'HEIL'E' BY ATTORN EY Patented Dec. 23, 1941 TELEVISION TRANSMITTING TUBE Richard Theile, Berlin, Germany, assignor to Telefunken Gesellschaft fiir Drahtlose Telegraphic in. b. H., Berlin, Germany, a corporation of Germany Application January 9, 1939, Serial No. 250,061

In Germany January 8, 1938' (Granted under the provisions of sec. 14, act of March 2, 1927; 357 0. G.

2 Claims.

In television transmitter equipment the oathode-ray tube has the purpose to convert or translate the photo image projected upon the picture screen into a sequence of electrical impulses coordinated to the distribution of brightness. The conversion of area and time in this scheme, i. e., the scanning operation, is effected by the electron-ray beam sweeping in the form of a raster over the picture screen, the beam in most instances serving as a switch for a secondary emission circuit, while the conversion from light to electricity is accomplished, for instance, by the use of an external photo-electric effect (Iconoscope) It has also been found that the conversion of light into electricity may be successfully efiected also by resorting to the utilization of resistance variations occasioned by illumination or irradiation by light 'of certain substances.

Another picture transmission tube has been disclosed in the art in which, by the use of the variation of the dielectric constant as a function of light actions (hereinafter briefly called the DC efiect) of an image screen made from a substance characterized by the said efiect (briefly called hereinafter the DC substance), the capacity element of the picture screen which happens to be in circuit, by way of a secondaryemission switch, is connected in parallel relation to the oscillation circuit of an RF transmitter of sufiiciently high frequency in such a way that frequency modulation of an RF carrier is obtained with a raster-pattern scan of the screen.

Now, the picture transmission tube here disclosed is predicated upon a totally different kind of conversion of light into electricity. Contradistinct from the arrangements that are known and disclosed in the earlier art, the production ofsignal potential in the picture transmitter tube of this invention is accomplished by the intermediary of a picture screen scanned by the cathode-ray pencil in which the photo image projected thereon results in a distribution of the time-constant corresponding to the picture content or subject-matter, formed by the capacity of the elementary section selected by the beam and the resistance of such section plus the secondary-emission inner resistance of the discharge path, the time-constant (for unilluminated screen) being suitably chosen in a very definite manner.

Advantages of the present invention will become apparent from the following specification and claims, especially when considered with the drawings wherein like reference characters represent like parts and wherein:

Figure 1 represents one embodiment of the present invention;

Figure 2 represents a detailed enlargement of a portion of Figure 1; and

Figures 3, 4 and 5 show operation curves of the invention.

The invention shall now be explained in greater detail by reference to the exemplified embodiment shown in Figure 1. B denotes the vessel of the picture transmitter tube. Included inside the same is the system G adapted to generate and focus the electron stream, the last (or second) anode R and a picture screen'E. A beam deflecting unit A is also provided. The tube is impressed with quiescent potentials determined by the sources of potential V and V connected in opposition. The picture screen consists of the following parts: Upon a metallic base or support P is brought a tenuous coat of a DC substance, say, a phosphor DC whose specific resistance has been chosen appropriately as herematter to be explained more fully. The DC substance preferably has .a photo conductivity effect such as exhibited by zinc cadmium sulphide.

What then results upon scanning,. as will be seen, is a picture or video signal of capacity controlled charge current pulses as hereinafter to be shown in greater detail.

The two-dimensional continum of the picture projection electrode for the sake of simpleness shall be conceived to be sub-divided into a multiplicity of elementary cells as illustrated in Figure 2. Each picture element or unit consists of an ohmic resistance r and a parallel capaci tance c. The beam moves from one element to the next in the course of a sweep. The assumption shall be made that the beam is just impinging upon the element marked 11. in Figure 2. According to the size of the ohmic resistance r and the special form of the secondary-emission characteristic, a certain drop of potential will arise, across the said resistance. It shall be presupposed that the secondary emission factor is greater than unity, and that the secondaryemission current 2}; flows in the signal circuit, that is, in the direction of the arrows. The drop of potential across the resistance co-ordinated to element 11. is i times 1. -If,-;then, the scanning beam sweeps over to the next element n+1, the drop of potential 12 times 1- will arise also in this case. However, this does not occur abruptly, or by leaps and bounds, but rather at a certain rate of speed inasmuch as the condenser at the.

which is governed only by the secondary-emission characteristic; After a sufficiently long time interval (a few micro-seconds) the. final value of the current 1;, conditioned by the inner resistance of the secondary emission discharge path and resistance r has arisen. If, then, the scanning time per picture element is chosen roughly of the same period as the decay constant of the current z' there results a shape as shown in Figure 4 for the scanning vof several elements.

If in the associated transmission system the details of the shape of the current inside the scanning time of a picture unit or element are lost there arises, in the ideal instance, a'mean value of current such as indicated by the dash-lines.

It will be understood that the continuous scanning, for an uninterrupted coat or film, of course,

' does not take place accompanied by such clearcut or definite switching-inpulses as shown in Figure 4; still, the foregoing considerations, basically, will hold also for such practically existing conditions. That is to say, in scanning the mean value of the signal current ip is higher than the value measured for the quiescent state.

The value of the mean current depends upon the speed of scanning or sweeping, or else, if the latter is stabilized, upon the time-constant of the decay process. This constant, in turn, is a function of the resistance and the capacity of a' picture element as well as ,the inner. resistance of the secondary emission path. By variation of the capacity of the element the discharge rate and thus also the size of the mean signalcurrent per element may be varied.

Now, for the production of the picture or video signal, use shall be made of the capacity variation upon illuminationof a suitably chosen or prepared DC substance, that is to say, a phosphor (called a DC phosphor). 'In other words, the capacity Cu. is a function of the light.

In order to realize what may be termed flight dependent decay control of the invention, one chief consideration is to make the time-constant of decay perelement, for dark capacity, roughly equal to thescanning period T of an element. This condition must be approximately fulfilled if maximum sensitivity. may be insured in picture transmission (pickup) tubes .as here disclosed. Of course, where demands are less severe as regards sensitiveness, the said condition may be more or less departed from.

In order to obey the said chief requirement, a number of measures according to the invention conducive thereto shall be outlined. One condition for satisfactory operation is that the conductivity of the phosphor should be at least so high that each elementary capacity will be enabled to become completely discharged inside the picture frequency (frame frequency).

For the decay curve, the following time-constant holds good:

where R is the inner resistance (here assumed to be linear) of the secondary-emission discharge path. If R is very high,'then t=c times 1' (approximately) and the innerresistance R is not materially effective; that is to say, in the presence of a definite inner resistance a very definite capacity (thickness of film) must be obtained and be present if optimal signals are to be secured. For reasons of the sharpness of the television picture the thickness or 'depth of the coat can not be chosen at will. Hence, it is suggested to secure the proper time-constant by proper choice of the ohmic, specific resistance of the layer of phosphor or else to obtain the same by suitable admixtures of a resistance material to the DC substance. According to the rate of scanning in terms of lines, an optimal signal strength may be furnished from the picture pickup tube by choice of suitable dimensions.

The biasing voltage V is chosen as high as feasible in the polarity indicated in Figure 1, though it should not be higher than the straight portion of the secondary-emission characteristic.

In brief, the invention is predicated upon a variation of the elementary time-constant of each picture element. In the exemplified embodiment so far discussed, variation of the said time-constant has been effected by photo-electrical variation of the elementary capacity. However, it would be just as readily possible to insure variation of the elementary time-constant by photoelectrical change of the elementary resistance, in other words, to coat the picture screenwith a layer or film whose specific resistance (resistivity) changes with the illumination. However, contradistinct to resistance scanners known in the art, the condition above laid down for the elementary time-constant must be fulfilled.

Iclaim:

1. A television transmitting tube including therein an electron gun structure adapted for generating a beam of electrons, and a planar target electrode in said envelope adapted to be scanned by the beam of electrons, including a signal plate upon which is deposited a layer of secondary electron emissive material, the resistance value of which varies in accordance with actinic radiation and upon which a light image may be projected, the thickness of the layer. being such as to produce a predetermined capacity between the outer surface of the, layer and the signal plate, whereby the resulting time constant in the absence of actinic irradiation will substantially equal the scanning time of each element thereof the time constant being variable in accordance with the intensity of the actinic irradiation projected thereon.

2. A television transmitting tube including a gun structure for producing a focused beam of electrons, a planartarget electrode in said envelope adapted to be scanned by the beam of electrons, said target electrode including a signal plate upon which is deposited a layer of high resistance material said material having a variable unit of resistivity depending upon the intensity of the light projected thereon, and a layer of discrete light responsive secondary electron emissive elements positioned on said resistance material, the capacity relationship between each light responsive element and the signal plate as well as the resistance of the material both increasing with actinic irradiation, the thickness of th layer of resistance material being so chosen that the time constant of the capacity relationship and the resistance of the high resistance material will substantially equal, the scanning time for each element in the absence of actinic irradiation and will increase upon an increase in actinic irradiation.

RICHARD THEILEJ." 

